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MG MGA - Super Market Fuels, MPG & Temperatures

I mentioned in a previous post that someone had told me that supermarket fuels returned poor MPG and that a long trip I made in my Merc appeared to support this rumour. I obtained an extra 4mpg with recognised fuel such as Shell.

Having now done a similar exercise in the MGA I have noticed also that my engine runs 5 degrees cooler with 98 octane Shell than 98 octane supermarket fuel.


Steve Gyles

Sorry Steve, can't come to terms with that. Perhaps some improvement may be evident in a modern high performance car where variable valve timing and every other modern gizmo can take full advantage of different octane levels, etc., etc. There are some tests recorded on the internet somewhere which support this.
But on an old style engine like an MGA it seems to me unlikely that different fuels would so significantly, if at all, affect engine temperature. Don't doubt for a minute your evidence, but having trouble with linking it with two different fuels of equal octane.
Just my view!
Bruce Mayo


It probably comes down to the additives that are added to the base fuel to acheive the required octane etc.

I was sitting next to a Shell executive on the way to Dubai the other week. He said that although the fuel for supermarkets come out the same tanks at the refinery, they put their own additives in (or not). Shell do not put their additives in other customer fuels.

I was down to almost the fumes of the supermarket fuel on Wednesday when I refuelled with Shell. I had been running at 190 for 10 miles to the garage, as I had for the previous 200 miles on the supermarket fuel. Refuelled to the brim; drove for another 8 miles with the temperature not over 185.

Yesterday and today temperature not above 185. In fact in my 10 years of MGA motoring the temperature rarely goes above 185. I have only once ever put supermarket fuel in and the temperature goes to 190. Sounds suspicious to me.

Steve Gyles

I tend to agree with Steve, the major oil companies can control their fuel, at the refinery and at the final franchised resellers.
We have had trouble in Australia in recent years with non-oil company resellers putting up to 25 percent of either ethanol, or worse still, 25 percent acetone in their fuel.
Following many complaints about poor performance and engine damage, both the oil companies and the Government now do random sampling of resellers' fuel for analysis.
It seems to have cured the problem.

Mick Anderson

Your post just reminded me of a caution I wanted to post.

This probably only applies in the US but there have been repeated problems with the undereducated Fuel Tanker driver mistakenly put e-85 in the regular fuel tanks.

The high alcohol content in e-85 melts the rubber parts in the fuel system and gums up the works.

I had to purchase a new fuel pump.

But here's the good news. Costco admitted that it was their fault and paid for it.

So be careful to go to reputable retailers.
T McCarthy

OK, so I'm being educated here! So do I now avoid my beloved Tesco 99 petrol - nice high octane which seems to suit the Twin Cam engine so well, and at a low price, and just around the corner - and start paying for Shell V Power or the like?
Bruce Mayo

About 15 years ago I was driving a 1987 Plymouth Duster. A little hatchback with a 5-speed and a 2.2-litre engine breathing through an electronically-controlled Holley carb. A trifle more elegant than an MGA engine perhaps, but not a whole lot. I found that running it on Mobil unleaded (not mid-grade or premium) vs Phillips 66 unleaded was cheaper per mile driven, although the gas cost about $.04 per gallon more. The improved MPG more than made up for the cost per gallon difference. Supermarket gas tends to be another level lower in the food chain.

Now that I am driving a 2002 VW GTi VR6 that tracks its fuel economy, I find that it also prefers Mobil over most other brands.
David "YMMV" Lieb
David Lieb

The people in my favourite engine workshop, where a lot of engines are built for drag racing and really powerful pony cars, are worried about the low quality of modern fuel in general. I would think that it is easier for modern engines with electronic management systems to adapt to varying fuel quality than for our old engines?



Yes, I think modern engines can handle a wider range of fuels. They have alloy cylinder heads for better heat dissapation, more accurately controlled coolant and oil temperatures, continuously adjusted fuel mixture based on feedback from the Lambda exhaust sensor, and automatic spark retarding if a reading is given by the engine "knock" sensor.


People in different parts of the world have different experiences with the fuel supplied by their reseller. If you are happy with your Tesco supermarket fuel, then stay with it.

Mick Anderson

This thread was discussed between 06/06/2008 and 09/06/2008

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